Padlocks are commonly used to lock a hasp having a slot through which there is a projecting staple. A prior art padlock typically includes a shackle or bow which fits through the loop of the staple and prevents the staple from removal through the hasp.
However, many shackles can be easily cut using commonly available tools. The traditional defense against this form of attack is to increase the diameter of the shackle legs, thereby increasing the size of the lock body, or by making the shackle from a hardened alloy steel in order to make cutting more difficult. However, this adds significantly to the cost of the padlock. Another problem is that the latching mechanisms and shackles are often vulnerable to wedging, prying, and torsional attack.
U.S. Pat. No. 4,548,058 to Dolev and Bahry describes a padlock, which is shown in FIG. 1. The padlock has two shackle protectors 4, which have intermeshing protrusions 2 and recesses 3. A locking module 5 is inserted into an inner volume of the protectors 4 (as indicated by arrow 1) and legs of a shackle 6 are inserted through an opening 9 of the protectors 4 into holes 7 of locking module 5. It is noted that shackle protectors 4 are locked together only when shackle 6 is locked with locking module 5. The smooth body of locking module 5 does not provide any locking or motion prevention action without shackle 6.